Published German Application 24 61 492 describes an apparatus and a process for separating particles in dependence on their coefficients of friction as the particles move under gravity on a helical chute from top to bottom. Each particle which has traveled over a minimum distance and has attained a minimum velocity will allegedly move on the helical chute at a constant radial distance from the axis of the helix, which distance will depend on the coefficient of friction of the particles, so that particles can be sorted in dependence on their coefficients of friction through a plurality of outlets which are distributed over the width of the chute. Particles having a high coefficient of friction allegedly move along narrower paths than particles having a lower coefficient of friction.
But the known process and the known apparatus still have severe disadvantages so that their economical utility is doubtful.
Because the particles are fed to the chute throughout its width, the number of particles which have a lower coefficient of friction and are initially received by the inner part of the chute will, on a statistical average, equal the number of particles which have a higher coefficient of friction and are initially received by the outer part of the chute. It will be appreciated that the paths along which said particles move will cross during the sorting operation and that the moving particles will necessarily influence each other so as to hinder the desired separation. That disadvantage is inevitable in the known process.
A further disadvantage of the known process resides in that a minimum number of convolutions is required for the helical chute but there is no upper limit for the number of convolutions. If a chute is selected which provides for a distance of travel that is distinctly larger than the minimum distance, which is not exactly defined, the result of the sorting operation will also increasingly be affected by the fact that the velocity of descent of the particles as well as their centrifugal acceleration increases and finally, when a sufficiently high velocity of descent has been attained, a further movement of all particles from the axis of the helix will finally be prevented only by the outer wall of the apparatus, regardless of the coefficients of friction of the particles. Whereas that disadvantage could be avoided in that an upper limit is defined for the distance of travel on the chute, such an upper limit just as the lower limit would have to be selected in view of the material composition of the particles and that measure would have the result that the known apparatus has a fixed overall size and a considerable expenditure would be involved in an adaptation to mixed materials having a different composition.